The present invention relates generally to the field of optical disc technology and more particularly, but without limitation, to the prevention of unauthorized duplication of an optical disc by providing changes in rates at which data pass a readback head assembly while the disc is rotated at a constant linear velocity.
Optical discs have become increasingly popular as efficient and cost-effective portable storage media for digitally stored data. A typical optical disc comprises a circular disc having a recording layer of light reflective material embedded in a refractive substrate. The recording layer is disposed along a plane substantially normal to an axis about which the disc is rotated and stores data as a sequence of optically detectable patterns, such as pits and lands (also sometimes referred to as xe2x80x9cmarksxe2x80x9d and xe2x80x9cspacesxe2x80x9d) along a continuously extending spiral track. The length of each pit and land corresponds to one of a selected number of data symbols (for example, from 3T to 11T, with T of determined length).
The data symbols are recovered from the disc through the use of a readback head assembly which includes a light source and an optical transducer. The light source applies light of selected wavelength to the rotating disc and the transducer generates a readback signal indicative of the data in relation to the relative differences in reflectivity of the patterns. Motor velocity control circuitry is used to rotate the disc at an appropriate linear velocity so that the rate (frequency) at which the patterns reach the readback head assembly is within the frequency lock capabilities of data recovery circuitry used to detect and decode the readback signal.
Optical discs typically store audio, video, computer programs and other related types of data. Present generation optical disc formats include compact disc (CD) and digital versatile disc (DVD), although other disc formats and configurations have been proposed and commercialized. While individual recordable discs are available to record data from a user device (such as a host computer), the vast majority of optical discs have historically been produced in a mastering process in which a master disc is generated. The master disc is used to form a series of stampers which are then used to create a population of injection molded replicated discs all having nominally the same characteristics. The large data capacity, low cost, convenience and portability of optical discs have contributed to the commercial success of such discs.
Due to the worldwide consumer demand for the types of data available on optical discs, combined with the relative ease with which unauthorized copies of optical discs can often be generated, suppliers of optical discs have attempted to implement various disc authentication schemes to authenticate the discs; that is, to determine whether a particular disc is an authorized copy. Such disc authentication can occur in the form of copy protection or forensic tracking efforts.
Generally, copy protection involves configuring the optical disc in such a manner that an authorized disc functions properly in a readback system, but an unauthorized copy does not. One common copy protection scheme used in computer data discs (e.g., CD-ROM) is to write certain copy protection data at certain locations on a disc. When the disc is subsequently loaded into a host computer, a system processor polls these locations and verifies the contents before granting access to remaining portions of the disc.
Forensic tracking efforts generally entail storing certain xe2x80x9cmarkingxe2x80x9d or xe2x80x9ctrackingxe2x80x9d information on the disc. The forensic information usually does not prevent an unauthorized copy from functioning in the readback system, but based on the presence or absence of the information, a determination can be made whether a particular optical disc is in fact an authorized copy. Forensic information may include, for example, information relating to the ownership of the contents of the disc, when and where the mastering process occurred, etc.
Although prior art disc authentication schemes have met with various levels of success, there remains a continued need for improved ways to prevent unauthorized duplication of optical discs, and it is to such improvements that the present invention is directed.
The present invention is directed to an apparatus and method for recording data to an optical disc to prevent unauthorized duplication of the disc.
In accordance with preferred embodiments, the data are written to the disc along a continuous spiral in the form of optically detectable patterns for subsequent access by a disc readback system. The readback system includes a motor configured to rotate the disc at a nominally constant linear velocity, a readback head assembly configured to transduce the patterns, and readback recovery circuitry configured to obtain frequency lock on the patterns as the patterns are rotated past the head assembly.
The disc is provided with a data placement zone to which first data are written to produce a first data rate as a frequency at which the first data pass the readback head assembly as the disc is rotated at a selected linear velocity. The disc further includes a velocity disruption zone adjacent the data placement zone to which second data are written to produce a second data rate as a frequency at which the second data pass the readback head assembly as the disc is rotated at the selected linear velocity.
Preferably, the variation in frequency between the first and second data rates is selected to cause a readback system to lose frequency lock on the second data during a continuous access operation on the first and second data. Thus, the first and second data can be accessed during separate access operations, but not during the same operation. This prevents a third party from creating an unauthorized duplicate of the disc through a continuous reading operation wherein the disc is read from lead-in to lead-out to generate a modulation signal which is then used to directly cut the duplicate disc.
In another embodiment, the disc is provided with a data profile zone to which information is stored relating to the first and second data rates, including a target disc linear velocity at which the disc is rotated to enable the readback system obtain frequency lock on and recover the second data. An unauthorized duplicate disc formed from the foregoing continuous read process will provide the second data at a different, nominal data rate as compared to the rate for an authorized disc.
Thus, during a disc access operation the disc linear velocity is measured as the readback system reads the second data. The measured disc linear velocity is compared to the target disc linear velocity, and further access to the disc is denied when a velocity mismatch is detected. In this way, even if a third party is able to create a bit-for-bit unauthorized duplicate of the original, the unauthorized duplicate will not exhibit the same velocity variations as the original and will not be playable in the readback system.
Other features and advantages which characterize the present invention will be understood by a review of the following detailed description section in conjunction with a review of the associated drawings.